The invention relates to a control strategy and method for truncating engine torque in a vehicle powertrain.
In contemporary automotive vehicle powertrains, it is usual practice to include a transmission with a gear system controlled by fluid pressure-operated clutches and brakes. The clutches and brakes, which are referred to herein as friction elements, are selectively engageable to establish multiple ratios as torque is delivered from a throttle-controlled internal combustion engine and through the transmission to vehicle traction wheels. The transmission clutches and brakes, which establish a reverse torque flow path through the gear system, are different than the clutches and brakes used to establish a forward-drive torque flow path.
The forward-drive clutches and brakes use a fluid pressure delivery path that includes calibrated orifices that are distinct from corresponding calibrated orifices in the pressure delivery path for clutches and brakes used to establish reverse drive. At cold ambient temperatures, when transmission fluid temperatures cause increased viscosity of the automatic transmission fluid, the clutch and brake disengagement time for one clutch, for example, may be longer than the clutch or brake engagement time for another clutch as the operator of the vehicle repetitively shifts the transmission between the forward-drive mode and the reverse-drive mode.
This maneuver commonly is referred to as cold rock cycling. It is used to extricate a vehicle from a low friction road surface, i.e., a snow, ice or mud-covered roadway. It is possible under such conditions for the applying friction element to gain torque capacity before a companion releasing friction element is disengaged or loses torque capacity. This may create a clutch tie-up event. If a clutch tie-up occurs when engine torque is at high or moderate levels, friction element wear or heat distress may occur.
Examples of multiple-ratio automotive transmissions having pressure-operated clutches and brakes in contemporary vehicle powertrains may be seen by referring to U.S. Pat. Nos. 6,122,583, 5,413,539, 5,039,305, 5,809,442, and 5,157,608, which are assigned to the assignee of the present invention. These patents are incorporated in the present disclosure by reference. Each of these patents discloses a transmission for an automotive vehicle powertrain wherein the fluid flow circuit for controlling a friction element for reverse drive is distinct from the fluid flow circuit for controlling a friction element for forward drive. Further, each of them discloses a transmission in which a manual valve, under the control of a vehicle operator, selects a forward-drive range or a reverse-drive range. The engagement-and-release pattern for the clutches and brakes of such transmissions is calibrated for optimum engagement and release time when the transmission fluid is at a controlled temperature.
The present invention reduces the possibility of friction element tie-up as the multiple-ratio automatic transmission is cycled between a forward-drive mode and a reverse-drive mode. When a ratio change is characterized by a high kinetic energy engagement, the invention is capable of reducing the possibility of heat distress in the clutch or brake friction material due to simultaneous high energy engagement of the clutches or brakes for one operating mode in one direction and the clutches and brakes that establish a torque delivery mode in the opposite direction. Indeed, the invention will protect the friction elements during any high energy engagement on a non-forward (R,P,N) to forward shift or a non-reverse (OD,P,N,D,2,1) to reverse shift.
The strategy of the present invention will establish a truncated or reduced engine torque at a specified level if the driving conditions are such that the strategy should be triggered due to operation of the clutches and brakes in a cold rock cycling mode. The strategy will be activated, for example, during repetitive high energy forward drive to reverse or reverse drive to forward engagements by reducing the engine torque if there is a possibility of a friction element tie-up condition.
Implementation of the strategy requires predetermined entry conditions before engine torque is reduced to a specified level. The entry conditions include those based upon engine rpm, throttle position, transmission fluid temperature, vehicle speed and the presence of power-on forward-to-reverse or reverse-to-forward engagements. The entry conditions are designed so that inadvertent flagging of the strategy will not occur when truncation of engine torque is not required. The magnitude of engine combustion torque reduction is a function of engine stability, engine temperature and the presence of a driver request, for example, a request for cold rock cycling. The duration of the engine torque reduction achieved by the strategy of the present invention is a function of automatic transmission fluid temperature.
A calibration function of torque versus engine speed is used to ensure that the requested torque is at a level sufficient to prevent engine stalling, misfires or engine instability.
A powertrain capable of using the strategy and method of the invention would include pressure-operated clutch and brake elements, engine coolant and transmission fluid temperature sensors, a traction wheel speed sensor, an engine throttle position sensor, and a driver-operated transmission range selector. The strategy includes the steps of determining the duration of a torque truncation when strategy energy conditions are met, verifying whether entry conditions are met, including verifying whether the transmission fluid temperature is below a threshold value, determining the magnitude of a torque truncation, and setting a maximum truncation value dependent on friction element capacity.